What's more, Ebola is much less contagious than many other more common diseases. The virus, much like HIV or hepatitis, is spread through blood or bodily fluids and is not airborne.

Many factors play into how contagious a disease is thought to be, say Jeff Duchin, MD, an infectious disease expert at the University of Washington, Seattle, and Amesh Adalja, MD, an infectious disease expert at the University of Pittsburgh.

Among those factors:

How it’s transmitted (airborne, bodily fluids, other)

Infection-control practices in place

Extent of contact an infected person has with others

Percent of the population that has been vaccinated (if a vaccine exists)

To gauge how contagious different diseases are, experts take these and other things into account and estimate the average number of people likely to catch the illness from a single infected person. They call this the basic reproductive rate or number. The number is an average, a scientific guess, experts say, and it is likely to vary from country to country.

"I would anticipate the reproductive rate for Ebola in the U.S. to be zero," Adalja says.

By comparison, measles, diphtheria, and whooping cough are all airborne, and they can be transmitted by "just being in face-to-face contact with an infected patient, without touching them,” Duchin says. When that person coughs or sneezes, others may become infected after breathing in the organisms.

Here are the estimated, overall, basic reproductive rates for Ebola and other infectious diseases, along with how they're spread.